1. Field of the Invention
The present invention relates to a method for preparing fuel-grade alcohol.
2. Description of the Prior Art
Ethanol as well as other alcohols is commonly prepared by fermentation of sugars or other biological feedstocks. In the fermentation process, the fermentable materials incuding yeast microorganisms are added to a large tank where fermentation is accomplished in a batch process. During fermentation the yeast cells or other microbes used consume the biomass feedstock in the tank and convert the feedstock to alcohol as they grow. The initial rate of fermentation is low, but then increases to a maximum rate which decreases again as the alcohol content in the fermentation medium increases. In fact, in the case of wine production, the relatively high alcohol content in the final product may be sufficient to actually kill the fermenting microorganisms.
After the fermentation has reached the desired stage of completion, the fermentation liquors are drained from the tank. Thereafter, if fuel-grade ethanol is to be recovered from the ferment, the fermentation liquors are clarified in a beer still and then fractionated to produce an ethanol-water azeotrope.
The conventional process for producing absolute ethanol has several drawbacks. One of the disadvantages is that the fermentation step is operated batchwise, which means that the average rate of conversion of fermentable material is lower than would be the case if the fermentation were conducted continuously. Consequently, the quantities of fermentable feedstocks must be maintained at high levels at the processing plant in order to maintain the desired production rate. Secondly, the distillation process for the recovery of ethanol is highly energy intensive. An analysis of the conventional alcohol distillation process shows that the total amount of energy required to obtain fuel-grade ethanol from the fermentation liquor is about 60% of the theoretical heating value of the ethanol product. This is a significant disadvantage for the large scale production of essentially water free ethanol for use as an ingredient in the production of gasoline - alcohol mixtures (commonly known as gasahol) which are to be used as motor fuels. If, in fact, ethanol is to find acceptable commercial utility as a motor fuel ingredient, the energy required to produce the substantially water free ethanol must be less than the energy required to recover substantially water free ethanol from fermentation liquors. Moreover, the conventional distillation and recovery process is complicated since it requires three distinct processing steps which are the (1) beer still, (2) the fractionator, and (3) the azeotropic distillation with benzene. A need, therefore, continues to exist for a method by which substantially water free alcohol, particularly ethanol, can be obtained using substantially less energy for the recovery of alcohol in comparison to conventional alcohol recovery procedures.